Reinforced Concrete I

WOLLO UniversityTECHNOLOGY Faculty

Department of Civil Engineering

Lecture-1

1

2

Introduction

Properties of reinforced concrete:

What is Concrete?

Concrete: -Concrete is stone like material obtained artificially by hardening of the mixture of cement, inert-aggregate materials (fine & course) and water in predetermined proportions.

3

Properties of reinforced concrete

The material called concrete in this Lecture is made three or four basic ingredients, usually called concrete making materials. These materials for concrete are:Hydraulic cement, usually portland

cementMineral aggregate(s)WaterAdmixture(s) (optional)

4

Properties of reinforced….

When these ingredients are mixed, they form a plastic mass which can be poured in suitable moulds (forms) and set-on standing into hard solid mass, as a result of exothermic chemical reaction between cement and water.

To produce a workable mix, more water is used over and above that needed for this chemical reaction.

5

Properties of reinforced….

Concrete has been the construction material used in the largest quantity for several decades.

The reason for its popularity can be found in the excellent technical properties of concrete as well as in the economy of this material.

It is also characteristic that the properties of concrete ingredients have a major influence on the fresh as well as hardened concrete. Therefore, the selection of concrete-making materials for a given purpose is quite important.

6

Properties of reinforced….

In order to make this selection intelligently, the selecting person should be able to assess

concrete-making materials, and should know what to select, how to select it, and why to select it in a particular way. In other words, he or she should be familiar with the available types of each of the concrete-making materials; the significance and application.

7

Compressive Strength of concrete:

A wide range of strength properties can be obtained for concrete by appropriate adjustment of the proportions of the constituent materials, using different degree of the compaction and the conditions of temperature and moisture under which it is placed and cured.

Water-cement ratio is the main factor affecting the strength of concrete, as shown in the next slide.

8

Compressive Strength of concrete:

 

9

Compressive Strength of concrete:

Standard test specimens of 150mm cube are taken at the age of 28days to determine the compressive strength of concrete according to Ethiopian standard institution (ESI).

At age of 7days, concrete may attain approximately about 2/3 of the full compressive strength of concrete.

In some national standard (example ACI code), cylinder specimens of 150mm diameter by 300mm high are taken.

10

Compressive Strength of concrete:

Although the load is applied uni-axially, the friction between the loading plate and the contact faces of the test specimen has more effect on cube strength than the cylinder strength.

Because of this, the cube strength gives more strength than the true compressive strength of concrete, whereas, cylinder strength gives reasonably the true compressive strength.

On average, cube strength is taken as 1.25 times cylinder strength.

11

Tensile strength of concrete

Even though concrete is weak in tension, its tensile strength is important in a variety of items.

Shear and torsion resistance of RC members primarily depend on tensile strength of concrete. Further, the conditions under which cracks form and propagate on tension zone of RC flexural members depend strongly on the tensile strength of concrete.

12

Tensile strength of concrete

Two methods are used to determine tensile strength of concrete. These are :

1. Beam-test and2. Split-cylinder test method.

According to EBCS-2, characteristic tensile strength of concrete is obtained using:

32)8.0(21.0 cuctk ff

13

Tensile strength of concrete

Beam test method, tensile strength of concrete is obtained by loading plain concrete test-beam laterally by two point loads at the third points of test-beam until the tension zone of the beam fracture.

Tensile strength of concrete is then computed using flexural stress formula

in terms of modulus of rupture concrete.I

cM .

14

Tensile strength of concrete

Split-cylinder test method, tensile strength of concrete is obtained by loading standard plain concrete cylinder along the side until the cylinder splits in to two pieces. The tensile strength of concrete is the computed by

based on the theory of elasticity for

homogeneous material in a bi-axial state of stress.

ld

P

..

2

15

Tensile strength of concrete

Whatever the method, it is known that, the tensile strength of concrete is relatively low, and it is about 10 to 15% of compressive strength of concrete.

16

Shrinkage and Thermal Movement

Concrete may under go deformations and volume changes with out application of loading. This phenomenon may be caused by shrinkage and thermal-movement in fresh and hardened concrete.

Shrinkage of concrete is liable to cause cracking, but it has the beneficial effect of strengthening the bond between the reinforcing steel and the surrounding concrete.

17

Shrinkage and Thermal Movement

Shrinkage of concrete caused initially by the absorption of water by cement and aggregate, and further by evaporation of water which rises to surface as a result of capillary action.

During setting process the hydration of cement causes a great deal of heat to be generated, and as the concrete cools, further shrinkage takes place due to thermal contraction.

18

Shrinkage and Thermal Movement

Thermal shrinkage may be reduced by: Using a mix-design with low cement

content. EBCS-2 specifies cement content not to exceed 550kg/m3 of concrete.

Avoiding rapid hardening & finely ground cement.

Keeping aggregate & mixing water cool, or may be need to keep them under shade.

Maintaining the temperature & evaporating water by proper curing.

19

Creep of Concrete

Creep is the continuous deformation of a member under sustained compressive stress over a considerable length of time (under long-term loading). It is a phenomenon associated with brittle materials (concrete is a brittle material).

Creep deformation depends on the stress in concrete, duration of loading and water-cement ratio.

The effect of creep has to be considered in design of reinforced concrete member subjected to compressive stress mainly caused by long term loading (dead load).

20

Creep of Concrete

The effect of creep is particularly important in beams, where the increased deformations may cause the opening of cracks and damage of finishes.

To reduce creep deformation, it is necessary to provide nominal reinforcement in the compression zone of the beam. The nominal area of compression steel required by doubly reinforced beam is about 0.4% of the area in compression (which may be taken as 0.2% of the whole area including tension zone).

Creep of Concrete

21

22

Reinforcing Steel

Steel reinforcements are available in the form of round bars and welded wire fabric.

The most commonly used bars have projected ribs on the surface of bar. Such bars are called deformed bars. The ribs of deformed bar improve the bond between steel and the surrounding concrete in RC members by providing mechanical keys.

23

Reinforcing Steel

A wide range of reinforcing bars is available with nominal diameter ranging 6mm to 35mm.

Most bars except 6mm diameter are deformed one. Some of the common bar size with their application in concrete works are given in table below.

Reinforcing Steel

24

For stirrups For slabs For beams & columnsDiam. (mm) 6 8 10 12 14 16 18 20 22 25 28

Area(cm2)

0.28 0.50 0.785 1.13 1.54 2.01 2.52 3.14 3.8 4.9 6.2

Weight

(kg/m)

0.222 0.395 0.617 0.888 1.21 1.57 2.0 2.47 3.0 3.9 4.8

Peri.(cm)

1.88 2.51 3.14 3.77 4.4 5.02 5.65 6.28 6.9 7.85 8.79

Not Available in the local market

25

Strength of reinforcing steel

Reinforcing steel is capable of resisting both tension and compression.

Compared with concrete, it is a high strength material. For instance, the strength of ordinary reinforcing steel is about 10 and 100 times, the compressive and tensile strength of common structural concrete.

 

Strength of reinforcing steel

26

27

Reinforced Concrete (as a composite material)

It is known that plain concrete is quite strong in compression, weak in tension.

On the other hand, steel is a high cost material which able to resist both tension & compression.

The two materials (plain concrete and reinforcing steel) are best be utilized in logical combination if steel bars are embedded in the plain concrete in tension zone close to the surface.

28

Reinforced Concrete (as a composite material)

In this case, plain concrete is made to resist the compressive stresses and reinforcing steel resists the tensile stresses.

Both plain concrete and reinforcing steel bar together assumed to act as one composite unit and it is termed as Reinforced concrete (RC).

The tensile stresses developed in the section are transferred to reinforcing steel by the bond between the interfaces of the two materials.

29

Reinforced Concrete (as a composite material)

In all RC members, strength design is made on the assumption that concrete does not resist any tensile stresses.

All the tensile stresses are assumed to be resisted by the reinforcing steel imbedded in tension zone.

Some times if necessary, reinforcing steel is provided in compression zone to assist the concrete resisting compression in addition to reducing creep deformation.

30

Reinforced Concrete (as a composite material)

Reinforcing steel and concrete may work readily in combinations due to the following reasons.

1. Bond between the bars and the surrounding concrete prevents slip of the bars relative to the concrete. Adequate concrete cover for steel bar and embedment length of bar are required to transfer stress between steel and concrete without slipping.

2. Proper concrete mixes provide adequate impermeability of concrete against bar corrosion.

3. Sufficiently similar rates of thermal expansion (0.00001/0C to 0.000013/0C for concrete and 0.000012/0C for steel) introduce negligible stresses between steel and concrete under temperature changes.

31

Advantages of Reinforced Concrete

It is monolithic. This gives it more rigidity. It is durable. It does not deteriorate with

time. While it is plastic, it can be moldable into

any desired shape. It is fire, weather and corrosion resistant. By proper proportioning of mix, concrete

can be made water-tight. Its maintenance cost is practically nil. 

32

Disadvantages of Reinforced Concrete:

It is difficult to demolish in case of repair of modification.

It is too difficult to inspect after the concrete has been poured.

33

Behavior of RC Beam under Lateral Loading

When beam is subjected to gradually increasing lateral load, there is change in stresses & deformations.

If these stresses and deformations exceed the capacity of the materials of the beam, the beam will fail.

Tests have shown that RC beams may fail either along a vertical (normal) plane or a diagonal plane. The aim of design of a member is to ensure resistance of section of beam along all planes.

34

Behavior of RC Beam under Lateral Loading

Three stages of behavior can be observed at a section of maximum moment, when singly reinforced beam is subjected to gradually increasing load till failure.

[See Handout,pp8-10]

Behavior of RC Beam… Stage I: Uncracked

35

Behavior of RC Beam… Stage II: Cracked Section Under Working Load

36

Behavior of RC Beam… Stage III: Cracked Section Under Ultimate Load

37

Design Philosophies (Methods)

38

The objective of reinforced concrete design is to achieve a structure or part structure that will result in a safe and economical solution.

For a given structural system, the design problem consists of the following steps:

1.Idealization of structure for analysis (dimension of members, support condition of structure and etc.)

2.Estimation of loadings.

39

Design Philosophies (Methods)

3. Analysis of idealized structural model to determine stress-resultants (axial forces, shear forces, torsions & bending moments) and their effects (deformations).

4. Design of structural elements (if assumed dimensions are adequate).

5. Detailed structural drawings and schedule of reinforcing bars.

40

Design Philosophies (Methods)

To achieve safe and economic structures, three philosophies of design had been adopted by codes of practices. These are:

1. Working Stress Design (WSD) or Elastic Design Method

2. Ultimate Strength Design (USD) Method, and

3. Limit State Design (LSD) Method.

41

Working Stress Design (WSD) method

WSD is the oldest and simplest method of design used for reinforced concrete structures.

It is based on the assumption that concrete is elastic, steel & concrete together act elastically. Also, the stresses developed in concrete & steel are not exceeded the respective allowable stresses any where in the structure when structure is subjected to the worst combination of service design loads.

42

Working Stress Design (WSD) method

The allowable stresses of materials are determined dividing material strengths by a factor of safety.

Safety factors specified by British standard are 3 for concrete and 1.8 for reinforcing steel. These safety factors are obtained from many years of practical experience and engineering judgment.

The safety factors specified by codes are assumed to cover all uncertainties existing in estimations of service design loads and material strengths.

43

Working Stress Design (WSD) method

The main drawbacks of WSD method are as follows:1. Concrete is not elastic material. The inelastic

behavior of concrete starts right from very low stresses. The actual stress distribution of concrete in section can not be described by a triangular stress diagram.

2. Since factor of safety is applied on the strength of materials, there is no way to account for different degrees of uncertainty associated with different types of loadings.

3. It is difficult to account for creep and shrinkage by computations of elastic stresses.

44

Limit State Design (LSD) method:

Limit state design method has developed from ultimate strength design method in order to apply in service load and ultimate load conditions.

Design of structure in limit state is made to achieve an acceptable probability that structure or part of it will not become unfit for use for which it is intended during expected life.

45

Limit State Design (LSD) method:

A structure with appropriate degrees of reliability should be able to withstand safely all possible combinations of design loads that are liable to act on it throughout its life and it should also satisfy the serviceability requirements, such as, limitations on deflection and cracking.

Further, it should be able to maintain the required structural integrity during and after accidents such as fires, explosions and local failure. In other words, all relevant limit states must to be considered in design to ensure an adequate degree of safety and serviceability.

46

Limit State Design (LSD) method:

These limit states which must be examined in design are broadly classified in to two major limit states. These are:1. Ultimate strength limit state (Limit state

of collapse), and2. Serviceability limit state

Ultimate limit state: the ultimate limit state is preferred to collapse.

Serviceability limit states: deflection, cracking, vibration, durability, fatigue, fire resistance and lightning.

47

Grade of Concrete:

Concrete is graded in terms of characteristic compressive cube strength.

The grade of concrete to be used in design depends on the classification of concrete works and its intended use.

EBCS-2 specifies grades of concrete for two classes of concrete works as shown below. Class Grades of concrete (MPa)

I C-5 C-15 C-20 C-25 C-30 C-40

C-50 C-60

II C-5 C-15 C-20

48

Design Strength of Material in Limit State:

The design strength for a given material and limit state is given by:

where fk -characteristic strength of

materials m –partial safety factor for materials

m

kd

ff

49

Design Strength for Concrete

(a)In compression: (ESCP-2/83)

(EBCS-2/95)

(b) In tension:

c

cucd

ff

67.0

c

cucd

ff

68.0

c

ctkctd

ff

50

Design Strength of Material in Limit State:

Idealized Stress-Strain Diagrams: For design purpose, most codes adopt idealized stress-strain diagrams in predicting the ultimate strength of sections in plastic-theory. In EBCS-2, a parabola-rectangle stress-strain diagram is given for concrete in compression as shown in figure below.

c

cucd

ff

68.0

Modulus of Elasticity of Concrete:

According to ESCP-2/83 and EBCS-2/95, mean value of the secant modulus, EC is given as shown in table below.

51

C-15 C-20 C-25 C-30 C-40 C-50 C-60

-ESCP-2/83 24 25 26 28 31 34 ---

-EBCS-2/95 26 27 29 32 35 37 39

)(MPafcu

)(GPaEc

Modulus of Elasticity of Concrete:

52

53

Design Strength for Steel

In tension and compression:

s

ykyd

ff

Design SituationsConcrete, c Reinforcing Steel, s

Class I Class II Class I Class II

Persistent and Transient 1.50 1.65 1.15 1.20

Accidental 1.30 1.45 1.00 1.10

Design Strength for Steel

54

EBCS,2: also idealized the stress-strain diagram for steel with ultimate strain of 0.01 as shown in figure below. It is a portion of stress-strain diagram of steel. The maximum strain of steel, permitted by code assumed to limit width of concrete crack in tension zone to acceptable limit.

s

ykyd

ff